Everyone knows that a dozen contains 12 items. But a baker’s dozen consists of 13 items. Baker adds 1 extra loaf to the bread. Perhaps the tradition continues from a law in medieval England. The law “The Assize of Bread and Ale”, passed in the reign of Henry III, regulated the price, weight and quality of bread. Any baker found overcharging customers was subject to harsh punishments, including fines, jail and beatings.
Since at that time bakers simply used loaves instead of scales for weight, they had to ensure they gave enough bread for the cost, so chose to add a bit extra, such as a whole loaf, to their dozen – preferring to take the loss than be flogged.